Fluid ink jet systems typically include one or more printheads having a plurality of ink jets from which drops of fluid are ejected towards a recording medium. The ink jets of a printhead receive ink from an ink supply chamber, or manifold, in the printhead which, in turn, receives ink from a source, such as a melted ink reservoir or an ink cartridge. Each ink jet includes a channel having one end in fluid communication with the ink supply manifold. The other end of the ink channel has an orifice, or nozzle, for ejecting drops of ink. The nozzles of the ink jets may be formed in an aperture, or nozzle, plate that has openings corresponding to the nozzles of the ink jets. During operation, drop ejecting signals activate actuators in the ink jets to expel drops of fluid from the ink jet nozzles onto the recording medium. By selectively activating the actuators of the ink jets to eject drops as the recording medium and/or printhead assembly are moved relative to each other, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium.
One difficulty faced by fluid ink jet systems is contamination of the exterior and/or interior ink pathways of a printhead. The exterior ink pathways of a printhead include the nozzle plate, ink jet nozzles in the nozzle plate, and the portions of the ink jet channels leading to the nozzles. The exterior ink pathways of a printhead may accumulate fibers, dust, and the like, during the printing process. In addition, excess dried ink may accumulate on the nozzle plate or in the nozzles and exterior channels of the ink jets. The accumulation of ink or other contaminants on the nozzle plate may partially or completely block the nozzles in the nozzle plate and, therefore, interfere with the passage of ink drops out of the nozzles.
The interior ink pathways include the ink supply, supply manifolds, ink supply pathways from the reservoirs to the manifolds, and ink jet channel inlets from the supply manifold to the ink jets. Interior ink pathways may be contaminated by particles, such as debris or gas bubbles. For example, debris may become trapped in a printhead during manufacture or assembly of the printhead. Gas or air bubbles may form in the interior ink pathways as a byproduct of operation of a printhead, such as, for example, high frequency firing of the ink jets or high operating temperatures in the printhead. These internal contaminants that form or originate in the interior ink pathways may accumulate at the ink jet channel inlets or enter into the channels and partially or completely block ink flow into the channels.
Partially or completely blocked ink jet nozzles and/or channels can lead to ink jet malfunctions or failures resulting in missing, undersized or misdirected drops on the recording media that degrade the print quality. Maintenance procedures have been implemented in ink jet printers for preventing and/or clearing ink jet blockages. Examples of such previously known maintenance procedures include purging and wiping.
Purging procedures typically involve ejecting a plurality of drops from each ink jet in order to clear contaminants from the jets. The purged ink may be collected in a waste ink reservoir, such as, for example, a waste tray or spittoon. Alternatively, ink may be purged onto an image transfer surface, such as, for example, a belt or drum, and subsequently cleaned from the transfer surface. Wiping procedures are usually performed by a wiper blade that moves relative to the nozzle plate to remove ink residue, as well as any paper, dust or other debris that has collected on the nozzle plate. Purging and wiping procedures may each be performed alone or in conjunction with each other. For example, a wiping procedure may be performed after ink is purged through the jets in order to wipe excess ink from the nozzle plate.
The ejection of the drops during a purging procedure may be controlled so that a purging operation may be effective against a particular form of ink jet contamination. For example, a purging procedure for clearing external contaminants from ink jet nozzles typically involves ejecting a plurality of drops in succession from each ink jet of a printhead. Ejecting a plurality of drops in succession from an ink jet may dislodge, and subsequently eject, contaminants that have accumulated in or around the ink jet nozzles.
A known purging procedure for clearing internal contaminants from the ink jet channel inlets involves firing the ink jets in a specific pattern to “move” internal contaminants that have accumulated at the channel inlets to less harmful positions in the manifold. The movement of the internal contaminants is caused by back pressure pulses that result from ink jet firings. The back pressure pulses may dislodge contaminants that have formed at the channel inlets and force them back into the manifold. By sequentially firing the jets, the sequential back pulses may push contaminants along the direction that the jets are fired until they reach less harmful positions within the manifold such as, for example, positions in the manifold where no jets are located.
Another known purging procedure that has been implemented to prevent or alleviate internal contamination of the ink jets comprises ejecting a plurality of drops from the ink jets at a lower firing frequency than a standard firing frequency for the jets. For example, when the ink jets are refilled with ink after firing a drop, the ink forms a meniscus in the corresponding nozzle. The meniscus behaves like a naturally damped membrane that seeks equilibrium undergoing simple harmonic oscillations. When the printhead assembly is operated at high frequencies, ink jets may be fired while the ink volume in the jet is still oscillating which may result in drops being ejected that vary in weight and velocity. Operating the printhead at lower frequencies is thought to stabilize the jetting by allowing the meniscus to return to a more natural or stable state.
In any case, printing must typically be stopped while a purging and/or wiping procedure is performed. In some previously known systems, printing may be stopped in the middle of printing a page to perform a maintenance procedure. While printing is stopped to perform maintenance, a significant amount of time may be expended. For example, each jet may be fired up to 100 times or more during a purging operation. Firing the jets in such a manner may take a few minutes to complete. If the ink is purged into a waste tray, time may also be expended in the positioning of the tray and/or printhead during the purging procedure. Wiping procedures also require print stoppage while the printhead and/or wiper blade are moved relative to the other. When wiping is used in conjunction with purging, the time expended for maintenance is even greater.
Stopping printing operations to perform a purging and/or a wiping operation decreases the printing time and, consequently, the throughput of a printer. Throughput is a rated characteristic, often measured in pages printed per minute. Consumers desire faster printers, and printers with a lower throughput rating are considered less desirable. In addition to the issue of time expenditure, maintenance procedures, purging in particular, may require a relatively significant amount of ink, e.g., 7-14 grams or more of ink per purging procedure. The purged ink cannot subsequently be used for printing purposes. As the number or frequency of purging procedures increases, the amount of printing that can be performed with a given volume of ink accordingly decreases.